[wellylug] Encrypted backup recovery

[jumbophut]

On Tue, 24 Aug 2004 18:00:29 +1200 (NZST), David Antliff wrote:
> 
> Encrypting a file can produce pretty much any kind of output. However it
> can't be completely random since it still contains information! 

Actually, there is a desire to have the encrypted file look random,
and good algorithms achieve this.  This is why some encryption is so
hard to crack.  It also means, as you rightly point out, that
encrypted files don't compress very well.  Bruce Schneier (Applied
Cryptography dude) has written about this and much other interesting
crypto stuff in his monthly crypto-gram newsletter:
<http://www.schneier.com/crypto-gram.html> (see the July 15 edition).

> It can also be shown that the 'arithmetic encoder' is the theoretical
> 'optimal' encoder and is already widely used. The advances in compression
> are in the area of models now - the better you can predict the next
> symbol, the less bits you need to use to encode it.
> 

I think 'near-optimal entropy encoding' is the expression used.  There
are unfortunately patents on arithmetic encoding, which prevents its
use in some open source applications (e.g. libjpeg uses Huffman
instead, even though the spec allows for arithmetic).  Anyhow, for
background, Shannon's seminal paper is a great read, even if it is
nearly 60 years old:
<http://cm.bell-labs.com/cm/ms/what/shannonday/shannon1948.pdf>. 
Clever, clever man.

Disclaimer: IANARCSAIRTRTBW (I am not a real computer scientist and I
reserve the right to be wrong).

-- 
Tony (echo 'spend!,pocket awide' | sed 'y/acdeikospntw!, /l at omcgtjuba.phi/')